An organic electroluminescent device (hereinafter referred to “organic EL device” when appropriate) is a planar light emitting device having a structure such that an organic material film is sandwiched between a pair of electrodes, which is characterized as being thin and lightweight, and having a wide viewing angle and a rapid responsibility, therefore, the organic EL device is expected to be widely used for various display devices. Recently, the organic EL device is being utilized for display of cell phones and others.
Organic EL devices are devices utilizing a function of emitting light upon recombination of holes, injected from an anode, with electrons injected from a cathode. Typical organic EL devices have a multilayer structure comprising a hole transport layer, an emitting layer, an electron transport layer and others. The hole transport layer and the electron transport layer are layers having a charge-transporting function, but not having a light-emitting function. More specifically the hole transport layer and the electron transport layer have a function of promoting charge-injection into the emitting layer, and confining charges injected in the emitting layer as well as energy of excitons generated in the emitting layer. Therefore the charge-transport layers play very important role for reducing the drive voltage of the organic EL device and enhancing the luminous efficiency.
As a hole transport material, amine compounds having appropriate ionization potential and hole transportability are used. For example, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPD) is well known as such amine compounds. However, NPD has a low glass transition temperature (Tg) and is readily crystallized under high temperature conditions, therefore, organic EL devices made by using NPD have a problem in durability. More specifically organic EL devices having a hole transport layer comprising NPD exhibit high drive voltage and low luminescent efficiency. Therefore, new improved materials are eagerly desired.
In recent years, an organic EL device having an emitting layer comprising a phosphorescent material is being developed. In the phosphorescent device, a hole transport material having a high triplet level is required. NPD is not satisfactory as the hole transport material for phosphorescent device. For example, it is reported that an organic EL device comprising NPD and a green phosphorescent material exhibits a low luminous efficiency (see, for example, non-patent document 1).
Recently, amine compounds having one or more carbazole rings in the molecule are reported as having a high triplet level as compared with that of NPD, and exhibiting good hole transportability. However, heretofore proposed amine compounds having a carbazole ring or rings have a chemical structure such that a nitrogen atom is bonded to 3-position of a carbazole ring (see, for example, patent documents 1 through 7). 3-Position of a carbazole ring is a para-position to the electron-donating nitrogen atom of the carbazole ring, therefore, an amino group introduced in 3-position of the carbazole ring is readily activated by the nitrogen atom of the carbazole ring. Thus, the amine compound having a carbazole ring or rings having an amino group introduced in 3-position of the carbazole ring or rings exhibits a low ionization potential as compared with that of the conventional amine compound. When the heretofore proposed amine compounds having a 3-amino-substituted carbazole ring or rings are used for a hole transport layer of an organic EL device, it becomes difficult to inject holes into an emitting layer, therefore, drive voltage of the organic EL device is inevitably increased.
In view of the foregoing background, an amine compound having a carbazole ring or rings having an amino substituent at 2-position of each carbazole ring is expected to exhibit an appropriate ionization potential. As examples of the amine compound having a 2-amino-substituted carbazole ring or rings, 2-ditolylaminocarbazoles are known as a charge transport material used in an organic photoconductor (see, for example, patent document 8). However, the 2-ditolylaminocarbazoles have a low glass transition temperature and, in the case when the 2-ditolylaminocarbazoles are used in an organic EL device, the organic EL device has a problem such that the durability is poor when the device is operated at a high temperature.
7-Phenyl-2-aminocarbazole compounds for use in organic electronic devices are proposed in, for example, patent document 9. Regarding these compounds having a chemical structure such that a phenyl group is bonded to 7-position of the carbazole ring, i.e., a para position to the amino group in the carbazole ring, since π-electron conjugation in the carbazole ring is extended, and the energy gap of molecule becomes small and the electron affinity is enhanced. Thus, in the case when the 7-phenyl-2-aminocarbazole compounds are used for a hole transport layer of an organic EL device, charges injected in the emitting layer and energy of excitons generated in the emitting layer become difficult to be confined in the emitting layer, and the luminous efficiency is lowered. Further, the 7-phenyl-2-aminocarbazole compounds exhibit a low triplet level, and therefore, an organic EL device comprising a phosphorescent material exhibiting a green light emission in combination with the 7-phenyl-2-aminocarbazole compounds do not exhibit a sufficient luminous efficiency.
Further, other types of carbazole compounds such as 4,4′-bis(9-carbazolyl)biphenyl and 1,3-bis(9-carbazolyl)benzene for use as a host material in an emitting layer are proposed in, for example, non-patent documents 2 and 3. These compounds have a high triplet level, but, exhibit high crystallinity and, when a thin film is formed therefrom, they are readily crystallized, and therefore, an organic EL device comprising these compounds have poor durability.